There are various types of fuel cells that are differentiated from each other by an ionic conductor, that is, a substance used as an electrolyte, and a solid oxide type fuel cell, SOFC (Solid Oxide Fuel Cell); herein referred to also as “a solid oxide fuel cell”, is a fuel cell using an oxide for a solid electrolytic material having ionic conductivity. This fuel cell generally has an operation temperature as high as on the order of 1000° C., however, there has lately been developed one having an operation temperature not higher than about 750° C. The solid oxide type fuel cell has the following features described under items (1) through (5) below:
(1) Because the electrochemical reaction at the electrodes proceeds smoothly due to a high operation temperature, the energy loss is low and power generation efficiency is high.
(2) Since the temperature of the waste heat is high, the power generation efficiency can be enhanced still higher by the multi-stage utilization of the waste heat.
(3) Since the operation temperature is high enough to cause hydrocarbon fuels such as natural gas to be reformed, it is possible to cause a reformation reaction to occur inside the fuel cell. In this respect, it is possible to implement substantial simplification of a fuel processing system (a reformer plus a shift converter) required in a low-temperature operation type fuel cell such as a phosphoric acid fuel cell, and polymer fuel cell.(4) As it is possible to cause carbon monoxide (CO) as well to contribute to the power generation reaction, the fuel can be diversified.(5) Since all constituent members are made up of solids, respectively, there is no risk of problems such as corrosion and evaporation of an electrolyte, occurring to a phosphoric acid fuel cell and molten carbonate fuel cell.
FIGS. 1 and 2 are schematic representations illustrating a form of a solid oxide type fuel cell by way of example. As shown in the figures, a fuel pole or anode and an air pole or cathode (an oxide pole in the case of oxygen being used as an oxidizing agent) are disposed with an electrolytic material sandwiched therebetween, and a single cell is made up of a triple-layer unit of fuel pole/electrolyte/air pole. For the electrolytic material, use is made of, for example, a sintered body in a sheet form, made of yttria-stabilized zirconia (YSZ), and so forth. For the fuel pole, use is made of, for example, a porous material made from a mixture of nickel and yttria-stabilized zirconia (Ni−YSZ cermet), and so forth. For the air pole, use is made of, for example, a porous material made from Sr-doped LaMnO3, and so forth. The single cell is made up normally by causing the fuel pole and the air pole to be backed into both-side faces of the electrolytic material.
When operating the solid oxide type fuel cell described above, fuel is fed to the fuel pole side of the single cell while air as an oxidizing agent is fed to the air pole side of the single cell, and by connecting both the poles to an external load, electric power can be obtained. However, with only one single cell, a voltage at only on the order of 0.7 V at most is obtained. Accordingly, it is necessary for obtaining electric power for practical use to connect a plurality of the single cells with each other in series.
For the purpose of electrically connecting adjacent single cells with each other; and properly distributing, and supplying fuel and air to the fuel pole and the air pole, respectively, at the same time, before discharging them, a separator (inter-connector) and the single cell are alternately laminated to each other. FIGS. 1 and 2 show a case where there are provided two single cells, with one separator interposed therebetween, and a frame body (serving as a kind of separator) is disposed on the upper surface of an upper single cell, and on the underside of a lower single cell, respectively.
FIG. 3 is a schematic representation illustrating a process of constructing a solid oxide type fuel cell using the separator described. As shown in FIG. 3, a current collector plate is disposed on the surface of the separator disposed on the underside of the fuel pole (Ni+YSZ), and on the upper surface of the separator disposed on top of the air pole, respectively, and by imposing a load from the current collector plate disposed on the upper surface of the separator disposed on top of the air pole, respective members are closely laminated with each other. In FIG. 3, there is shown a case where one single cell made up of the triple-layer unit is used, however, the same applies to a case where a plurality of single cells are disposed so as to be laminated to each other.
With reference to the separators to be used as described above, numerous properties as described under items (1) through (8) are required:
(1) dense enough not to allow gas to pass, and to leak therethrough;
(2) electron conductivity is high;
(3) ionic conductivity is low enough to be negligible;
(4) the constituent material itself is chemically stable in both an oxidizing atmosphere and a reducing atmosphere at a high temperature;
(5) there occurs no reaction thereof with other constituent members such as the two poles, and no excessive mutual diffusion therewith;
(6) the thermal expansion coefficient thereof matches those of other constituent materials of the cell;
(7) changes in size, due to a variation in atmosphere, are small; and
(8) sufficient strength.
Because of such severe requirements, there is a limitation to the constituent material that can be used as the separator under an operating condition close to 1000° C. As a constituent material meeting those requirements as much as possible, use is most generally made of an oxide solid solution of a LaCrO3 group (lanthanum chromite). With the constituent material described, a portion of the La is replaced with an alkaline earth metal such as Ca, Sr, etc., and further, a portion of the Cr is replaced with a 3d transition metal element such as Mg, Co, Mn, Ni, etc., thereby optimizing the properties thereof so as to meet those described requirements.
Now, with the solid oxide type fuel cell operating at a temperature not higher than about 750° C., there has been proposed the use of an alloy such as a heat-resistant alloy containing chromium as the constituent material for the separator. Even in the case of such a material for use in the separator as described, the separator is naturally provided with grooves to allow the passage of air and fuel.
FIG. 4 is a schematic representation illustrating the construction of the conventional separator described as above by way of example. As shown in FIG. 4, there is the need for providing a plate body with a plurality of grooves, and machining is essential for forming such grooves in an alloy such as the heat-resistant alloy containing chromium. Machining of the alloy described, however, is very difficult because the alloy has a high hardness and partial cutting is required, eventually resulting in a high cost. This point poses a problem in putting it to commercial use. Further, since the metal (alloy) has a high thermal expansion coefficient in comparison with a cell made of a ceramic, use of a separator made of a metal causes stress to occur to a cell due to the rise and fall in temperature, posing a problem of breakage occurring to the cell.
In view of the fact and problems as described above in connection with the flat plate type solid oxide fuel cell, the invention has been developed in order to resolve those problems. It is therefore an object of the invention not only to eliminate the above-described problem with the processing and the problem of thermal stress cracking by introducing a novel idea to the separator made of the alloy, such as the heat-resistant alloy containing chromium, for use in the flat plate type solid oxide fuel cell but also to provide a laminated structure of a flat plate type solid oxide fuel cell, having excellent performance, in terms of performance as the flat plate type solid oxide fuel cell using the separator, and the separator to enable the object to be attained.